Bycatch vulnerability
Seabirds feeding on the sea
For seabirds, fishing boats offer "free" buffets on the sea. Free food for some; for others, risky bait-taking interactions may cost their lives.
Photo credit: Graham Robertson

Seabird bycatch vulnerability in pelagic longline fisheries based on modeling of a long-term dataset by Can Zhou and Nigel Brothers


Background The incidental mortality of seabirds in fisheries remains a serious global concern. Obtaining unbiased and accurate estimates of bycatch rates is one of the priorities for seabird bycatch mitigation and demographics research. For measuring the capture risk of seabird interactions in fisheries, carcass retrieval rate from hauled gear is commonly used.

Problem statement However, reliability can be limited by a lack of direct capture observations and the substantial pre-haul bycatch losses known to occur, meaning incidence of seabird bycatch is underestimated.

Contribution To solve this problem, a new measure (i.e., bycatch vulnerability) that links an observed interaction directly to the underlying capture event is proposed to represent the capture risk of fishery interactions by seabirds. The new measure is not affected by subsequent bycatch loss. To illustrate how to estimate and analyse bycatch vulnerability, a case study based on a long-term dataset of seabird interactions and capture confirmation is provided. Bayesian modelling and hypothesis testing were conducted to identify important bycatch risk factors. Competition was found to play a central role in determining seabird bycatch vulnerability. More competitive environments were riskier for seabirds, and larger and thus more competitive species were more at risk than smaller sized and less competitive species. Species foraging behaviour also played a role. On the other hand, we did not find any evidence on the additional effect of physical oceanic condition and spatio-temporal factors.

Recommendations Bycatch vulnerability is recommended as a replacement for the commonly used bycatch rate or carcass retrieval rate to measure the capture risk of an interaction. Combined with a normalized contact rate, bycatch vulnerability offers an unbiased estimate of seabird bycatch rate in pelagic longline fisheries.

This is the second publication in this series of studies to unravel the difficulties in documenting seabird bycatch in pelagic longline fisheries. Oddly enough, this series started at examining the final stage of the seabird bycatch process first: seabird bycatch loss, and gradually moved forward. This is no coincidence. Most of the fisheries observer programs only document bycatch at the final stage of the fishing operation when captured animals are hauled onboard the vessel. As a consequence, those captured throughout the fishing operation but failed to be hauled back to the vessel are essentially lost. GONE. Such biological monitoring programs will systemactically underestimate the impact of commercial fisheries to ecosystems.

On average 50% of the captured seabirds failed to be observed at the final stage. However, bycatch loss (either as a process or as a parameter) holds no conservation value per se, since the damage is already done whether or not the bycaught seabird can be recorded by the observer. What is hiding behind bycatch loss is the bycatch vulnerability \(p_c\), which measures the capture probability of a bait-taking interaction, and it is the focus of this study. Basically, bycatch loss \(p_{loss}\) and bycatch vulnerability \(p_c\) are two sides of the same coin, except that only one side (\(p_c\)) is of paramount conservation significance with the other side (\(p_{loss}\)) being an artifact of the traditional haul-only observer protocol.

The last paragraph or the last sentence pretty much summarised the whole idea of the paper, and the rest of the paper is just technicalities and some facts. Orignially, the manuscript was sent to Biologcal Conservation, but it was swiftly rejected editorially for being quite similar to our previous publication there, a decision we anticipated and gracefully accepted. In our defense, those two papers (doi:10.1017/S0959270921000046 and doi:10.1016/j.biocon.2020.108590) are NOT similar, they are two sides of the same process!

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